BMB 601 MRI. Ari Borthakur, PhD. Assistant Professor, Department of Radiology Associate Director, Center for Magnetic Resonance & Optical Imaging

Transcription

1 BMB 601 MRI Ari Borthakur, PhD Assistant Professor, Department of Radiology Associate Director, Center for Magnetic Resonance & Optical Imaging University of Pennsylvania School of Medicine A brief history Isidor Rabi wins Nobel prize in Physics (1944) for for his resonance method for recording the magnetic properties of atomic nuclei. Felix Bloch and Edward Purcell share Nobel Prize in Physics (1952) for discovery of magnetic resonance phenomenon. Richard Ernst wins Nobel Prize in Chemistry (1991) for 2D Fourier Transform NMR. His post-doc, Kurt Wuthrich awarded a Nobel Prize in Chemistry (2002) for 3D structure of macromolecules. Nobel Prize in Physiology & Medicine awarded to Sir Peter Mansfield and Paul Lauterbur (2003) for MRI. Who s next? Seiji Ogawa for BOLD fmri? slide 2 1

2 Outline Slide # 3 Hardware: MRI scanner RF coil Gradients Image contrast Software: Pulse sequences Fourier Transform slide 3 How does it work? slide 4 2

3 Original Concept* 1. Rotating Gradient 2. Filtered Back-projection *Lauterbur, Nature (1973) slide 5 Fourier Imaging* 1. Three orthogonal gradients: 2. k-space Slice Selection Phase Encoding Frequency Encoding Representation of encoded signal *Kumar, et al. J. Magn. Reson. (1975) slide 6 3

4 MRI scanners 19 Tesla 4.7 Tesla 1.5 Tesla To generate B 0 field and create polarization (M 0 ) slide 7 MRI coils Mouse MRI platform Torso coil Head coil To transmit RF field (B 1 ) and receive signal slide 8 4

5 Gradients To spatially encode the MRI signal slide 9 Image Contrast Contrast in MRI based on differences in: Relaxation times (T 1, T 2, T 2*, T 1ρ ) Local environment Magnetization = spin density Concentration of water, sodium, phosphorous etc. Macromolecular interactions Chemical-exchange, dipole-dipole, quadrupolar interactions Contrast agents Gadolium-based, iron-oxide, manganese slide 10 5

6 Magnetization spin spin ensemble spin-up or spin-down = Boltzmann distribution Bitar et al. RadioGraphics (2006) Net Magnetic Moment slide 11 Energy levels Boltzmann distribution N + /N - = exp (-ΔE/k B T) Energy gap Larmor Equation slide 12 6

7 MR experiment RF pulse applied in x-y transverse plane RF off M 0 aligned along B 0 M z =M 0 M nutates down to x-y plane M z =M 0 cosα and M xy =M 0 sinα Detect M xy signal in the same RF coil slide 13 Equation of motion Bloch Equation (simple form) slide 14 7

8 Effect of RF pulse The 2 nd B field Choosing a frame of reference rotating at ω rf, makes B 1 appear static: slide 15 Nutation Lab frame of reference e.g. B 1 oscillating in x-y plane Rotating frame of reference e.g. B 1 along y-axis ~brian/mri-movies/ slide 16 8

9 After RF pulse-relaxation Spin-Lattice relaxation time=return to thermal equilibrium M 0 Spin-spin relaxation without reversible dephasing Spin-spin relaxation with reversible dephasing T 1 >T 2 >T 2 * slide 17 Signal detection ~brian/mri-movies/ slide 18 9

10 How does MRI work? NMR signal MR Image? = Water Fat 4.7 ppm 1.2 ppm 1) Spatial encoding gradients 2) Fourier transform slide 19 Pulse Sequence (timing diagram) Less MRI time/low image quality Less MRI time/low image quality RF slice phase freq RF slice phase TE TR Spin-echo α (<90 ) TE RF slice phase freq RF slice phase TR Fast spin-echo 90 TE freq TR Gradient-echo freq Echo planar imaging slide 20 10

11 MRI B 0 M 0 B 1 G z G y G x slide 21 Slice Selection* Gradient-echo α RF slice TE phase freq TR *Mansfield, et al. J. Magn. Reson. (1976) *Hoult, J. Magn. Reson. (1977) slide 22 11

12 Slice Selection γg z z B 0 Δz{ M z M z xy RF M z BW RF = γg z Δz slide 23 Frequency Encoding* a.k.a readout Gradient-echo α RF slice TE phase freq TR *Kumar, et al. J. Magn. Reson. (1975) slide 24 12

13 Frequency Encoding γg x x M z FOV x BW read = γg x FOV x slide 25 Phase Encoding* Gradient-echo α RF slice TE phase freq τ pe TR *Kumar, et al. J. Magn. Reson. (1975) *Edelstein, et al. Phys. Med. Biol. (1980) slide 26 13

14 Phase Encoding γg y2 y FOV y γg y1 y γg y0 y 1/τ pe = γδg y FOV y Each PE step imparts a different phase twist to the magnetization along y slide 27 Traversing k-space Spin-warp imaging* k y RF slice phase freq α Gradient-echo TE TR k x *Edelstein, et al. Phys. Med. Biol. (1980) slide 28 14

15 Fourier Transform k-space image FT freq phase slide 29 Traversing k-space Spiral Radial Echo-planar freq phase freq Spiral freq freq Echo planar freq Radial slide 30 15

16 Fourier Transform slide 31 Typical FT pairs slide 32 16

17 K-space The signal a coil receives is from the whole object: ( ) = ρ x,y S t x,t y ( )e iγ ( G xt x x+ G y t y y ) dxdy replace: k x, y = γg x, y t x, y K-space signal: ( ) = ρ x,y S k x,k y ( )e iγ ( k x x+ k y y) dxdy The image is a 2D FT of the k-space signal*: ( ) = S k x,k y ρ x,y ( )e iγ ( k x x+ k y y ) dkx dk y *Kumar, et al. J. Magn. Reson. (1975) slide 33 K-space weighting High-frequency data=edges Low-frequency data=contrast/signal slide 34 17

18 What is wrong? Original Freq. de-phaser too large! *Pauly, slide 35 What is wrong? Original k y step was too large or FOV y was too small! slide 36 18

19 What is wrong? Original Freq. gradient amplitude was too low! slide 37 MRI Examples slide 38 19

20 3D Neuro MRI slide 39 Susceptibility-Weighted Imaging (SWI) slide 40 20

21 2/3/11 BOLD fmri slide 41 Phase Contrast MR Angiography slide 42 21

22 TSE TE = 25 ms TSE Fat Saturated Resolution:200x200µm 2 slide 43 slide 44 22

23 Rationale for Sodium MRI [PG] decreases with Degeneration Urban et al Spine 1998 slide 45 Rationale for Sodium MRI Sodium is a natural biomarker for PG Urban and Maroudas Bioc. Biop. Acta 1979 Urban and Winlove, J. Magn. Reson. Imag slide 46 23

24 How is [Na] related to [PG]? Fixed Charge Density (FCD) Side chains of PG [Na] correlated with [PG] through [FCD] Maroudas et al., in Adult Articular Cartilage (1979) Lesperance et al., J. Orthop. Res. (1992) slide 47 Sodium MRI What do you need? 1. RF coil 2. Transmit/receive switch 3. Broadband capable scanner (all vendors) 4. A short echo-time MRI pulse sequence slide 48 24

25 2/3/11 Sodium MRI Ex Vivo Sodium maps in bovine discs Wang et al., Spine (2010) slide 49 Sodium MRI Ex Vivo Sodium content vs. PG content Wang et al., Spine (2010) slide 50 25

26 2/3/11 Sodium MRI In Vivo Sodium mapping vs. T2 MRI Wang et al., Spine (2010) slide 51 New 7T MRI! ultra-short echo pulse sequence TE/TR=200µs/25ms 2mm isotropic 15 minute acquisition slide 52 26

27 Sodium MRI Movies Axial Coronal Sagittal slide 53 Sodium MRI Montage SNR ~26:1 in cartilage slide 54 27

28 Magnet Create B 0 Take home Slide # 55 Produce M 0 RF coil Transmit B 1 field Detect Signal Image contrast Relaxation, concentration, interactions etc. Gradients Spatial encoding Signal in k-space Fourier Transform From k-space to image space Pulse sequences Traverse k-space Image Artifacts slide 55 28